Bacteriology — MCQs

Bilateral tonsillectomy is performed on an otherwise healthy 11-year-old female with recurrent upper respiratory tract infections. On pathological examination of the tonsils, numerous small, yellow granules are noted. A granule crushed between two slides reveals a dense, gram-positive center and numerous branching filaments at the periphery. The granules are most likely composed of which of the following organisms?

Q1287Easy

How are diphtheria carriers diagnosed?

Q1288Easy

Malignant pustules are characteristic of which type of infection?

Q1289Easy

Stalactite growth in a ghee broth culture is characteristic of which bacterium?

Q1290Easy

What is the most common sensitive causative organism for Traveller's diarrhoea?

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1281: Which of the following is true about H. pylori?

A. Produces Vacuolating cytotoxin (Correct Answer)
B. Antiurease antibody detection is specific for type I strains
C. Inhabits the jejunum
D. Treatment involves Al(OH)2

Explanation: **Explanation:** *Helicobacter pylori* is a microaerophilic, Gram-negative spiral bacterium primarily associated with peptic ulcer disease and gastric carcinoma. **Why Option A is Correct:** *H. pylori* possesses several virulence factors, the most significant being the **Vacuolating cytotoxin (VacA)** and the **Cytotoxin-associated gene A (CagA)**. VacA induces the formation of large cytoplasmic vacuoles in gastric epithelial cells, leading to cell death and tissue damage. Strains expressing both VacA and CagA (Type I strains) are more strongly associated with severe inflammation and malignancy. **Why Other Options are Incorrect:** * **Option B:** While Type I strains are more virulent, **Anti-CagA antibodies** (not antiurease) are the specific markers used to identify these aggressive strains. Urease is produced by almost all *H. pylori* strains to neutralize gastric acid; therefore, antiurease antibodies lack specificity for Type I. * **Option C:** *H. pylori* specifically colonizes the **gastric mucosa** (primarily the antrum). It does not inhabit the jejunum, as it requires the specific pH environment and mucosal receptors found in the stomach. * **Option D:** Treatment involves **Triple Therapy** (Clarithromycin, Amoxicillin/Metronidazole, and a Proton Pump Inhibitor). Al(OH)₃ is an antacid used for symptomatic relief but has no curative or antimicrobial effect against the infection. **High-Yield Clinical Pearls for NEET-PG:** * **Urease Breath Test:** The non-invasive "Gold Standard" for monitoring treatment success. * **Biopsy Urease Test (RUT):** A rapid invasive test (color change from yellow to pink). * **MALToma:** *H. pylori* is a Class I carcinogen; its eradication can lead to the regression of low-grade gastric MALT lymphoma. * **Culture:** Uses Skirrow’s medium or Chocolate agar.

Question 1282: True about V. Cholerae is:

A. One attack of V. Cholerae gives lifelong immunity.
B. Affects adults and children with equal propensity in non-epidemic regions.
C. Affects adults and children with equal propensity in non-epidemic regions.
D. Pathogenicity of O-139 Vibrio is due to O antigen. (Correct Answer)

Explanation: ### Explanation **Correct Option: D. Pathogenicity of O-139 Vibrio is due to O antigen.** The pathogenicity of *Vibrio cholerae* O139 (Bengal strain) is unique because it possesses a specific **O-antigen polysaccharide** and a **capsule**, both of which are essential for its virulence. Unlike the O1 serogroup, O139 has a distinct lipopolysaccharide (LPS) structure that allows it to cause epidemic cholera even in populations with pre-existing immunity to O1. **Analysis of Incorrect Options:** * **Option A:** Immunity following a natural infection with *V. cholerae* is **not lifelong**. While it provides significant protection against the same serogroup for approximately 3–10 years (longer for El Tor than Classical), it does not last a lifetime. * **Options B & C:** In **endemic (non-epidemic) regions**, cholera primarily affects **children**. This is because adults in these areas have usually developed acquired immunity through repeated subclinical exposures. In contrast, in **epidemic/pandemic** situations (where the population is "immunologically naive"), both adults and children are affected with equal propensity. **High-Yield Clinical Pearls for NEET-PG:** * **Serogroups:** Only O1 and O139 cause epidemic cholera. O1 is further divided into biotypes (Classical and El Tor) and serotypes (Ogawa, Inaba, Hikojima). * **Mechanism:** Pathogenicity is primarily mediated by the **Cholera Toxin (Choleragen)**, which increases intracellular **cAMP**, leading to the hypersecretion of water and electrolytes (Rice-water stools). * **Culture:** **TCBS (Thiosulfate Citrate Bile Salts Sucrose)** agar is the selective medium of choice, where *V. cholerae* produces large yellow colonies due to sucrose fermentation. * **Transport Media:** Venkatraman-Ramakrishnan (VR) medium or Cary-Blair medium are used for stool samples.

Question 1283: Which is not a typical mechanism of action of obligate anaerobes for producing energy?

A. Produce energy by exploiting the cytochrome system (Correct Answer)
B. Absence of air enhancing their growth potential
C. Lack of the enzyme catalase
D. Devoid of superoxide dismutase

Explanation: ### Explanation **1. Why Option A is the Correct Answer (The Underlying Concept):** Obligate anaerobes are organisms that cannot grow in the presence of oxygen. The **cytochrome system** (Electron Transport Chain) is the hallmark of aerobic respiration, where oxygen acts as the final electron acceptor to produce ATP. Obligate anaerobes **lack a cytochrome system** and instead generate energy through **fermentation** or anaerobic respiration (using inorganic molecules like sulfate or nitrate as electron acceptors). Therefore, exploiting the cytochrome system is not a mechanism used by these organisms. **2. Analysis of Incorrect Options:** * **Option B (Absence of air enhancing growth):** This is a defining characteristic. In the presence of oxygen, reactive oxygen species (ROS) are formed. Since anaerobes cannot neutralize these, they thrive only in the absence of air (low redox potential). * **Option C & D (Lack of Catalase and Superoxide Dismutase):** These are the primary reasons for their "obligate" nature. Aerobic metabolism produces toxic byproducts like superoxide ($O_2^-$) and hydrogen peroxide ($H_2O_2$). Obligate anaerobes lack **Superoxide Dismutase (SOD)**, **Catalase**, and **Peroxidase**, making them susceptible to lethal oxidative damage. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Key Examples:** *Clostridium* (Gram-positive bacilli), *Bacteroides* (Gram-negative bacilli - most common in intra-abdominal infections), and *Peptostreptococcus* (Gram-positive cocci). * **Redox Potential ($E_h$):** Anaerobes require a low $E_h$ environment. Tissue injury, necrosis, or stasis lowers $E_h$, facilitating anaerobic infections. * **Clinical Clues:** Foul-smelling discharge (due to fermentation products), gas in tissues (crepitus), and infections near mucosal surfaces. * **Culture:** They are typically grown in **Robertson’s Cooked Meat (RCM) broth** or using an **Anaerobic Jar (GasPak system)**.

Question 1284: 'Bull neck' in Diphtheria is due to which of the following?

A. Retropharyngeal abscess
B. Laryngeal edema
C. Cellulitis
D. Lymphadenopathy (Correct Answer)

Explanation: **Explanation:** The hallmark clinical feature of **Diphtheria** (caused by *Corynebacterium diphtheriae*) is the formation of a greyish-white pseudomembrane. The characteristic **'Bull neck' appearance** is primarily due to massive **cervical lymphadenopathy** and associated periadenitis (inflammation of the tissues surrounding the lymph nodes) and edema in the soft tissues of the neck. **Analysis of Options:** * **D. Lymphadenopathy (Correct):** The diphtheria toxin causes local tissue destruction and intense inflammation. This leads to significant enlargement of the cervical lymph nodes. When combined with surrounding soft tissue edema, it obliterates the angle of the jaw, resulting in the "Bull neck" deformity. * **A. Retropharyngeal abscess:** While this can cause neck swelling, it is typically a complication of pyogenic infections (like *S. aureus*) or TB (Pott’s spine), not the primary cause of the classic diphtheritic bull neck. * **B. Laryngeal edema:** This occurs in laryngeal diphtheria and leads to airway obstruction and stridor, but it does not manifest as external "bull neck" swelling. * **C. Cellulitis:** Although there is inflammation of the subcutaneous tissues (periadenitis), the primary pathological driver and clinical descriptor for this specific sign is the profound lymph node involvement. **High-Yield Clinical Pearls for NEET-PG:** * **Schick Test:** Used to demonstrate immunity/susceptibility to diphtheria. * **Culture Media:** Loeffler’s Serum Slope (rapid growth) and Potassium Tellurite agar (black colonies). * **Microscopy:** Gram-positive bacilli showing **Chinese letter pattern** (cuneiform) and **Volutin/Metachromatic granules** (Albert’s stain). * **Virulence:** Mediated by an exotoxin (fragment A inhibits Protein Synthesis via EF-2). * **Complications:** Myocarditis (most common cause of death) and neurological deficits (palatal paralysis).

Question 1285: The Weil-Felix reaction with OXK antigen indicates infection with which of the following organisms?

A. Rickettsia tsutsugamushi (Correct Answer)
B. Rickettsia mooseri
C. Rickettsia quintana
D. Rickettsia akari

Explanation: ### Explanation The **Weil-Felix reaction** is a heterophile agglutination test used for the presumptive diagnosis of Rickettsial diseases. It relies on the cross-reactivity between antibodies produced during certain rickettsial infections and the somatic (O) antigens of specific strains of **Proteus vulgaris (OX19, OX2)** and **Proteus mirabilis (OXK)**. **1. Why Option A is Correct:** * **Rickettsia tsutsugamushi** (now *Orientia tsutsugamushi*), the causative agent of **Scrub Typhus**, is the only major rickettsial pathogen that reacts specifically with the **OXK** antigen. It does not react with OX19 or OX2. **2. Why the Other Options are Incorrect:** * **Option B: Rickettsia mooseri** (now *R. typhi*) causes Endemic (Murine) Typhus. It shows a strongly positive reaction with **OX19** and a weak/negative reaction with OXK. * **Option C: Rickettsia quintana** (now *Bartonella quintana*) causes Trench Fever. It is **Weil-Felix negative**, as are most Bartonella species. * **Option D: Rickettsia akari** causes Rickettsialpox. Along with *R. prowazekii* (Epidemic typhus), it typically shows a negative Weil-Felix reaction (or very low titers), making the test unreliable for these specific infections. **High-Yield Clinical Pearls for NEET-PG:** * **Scrub Typhus (OXK+):** Transmitted by the bite of a **larval mite (chigger)**; characterized by a pathognomonic **black eschar**. * **Epidemic Typhus (OX19+):** Caused by *R. prowazekii*; transmitted by the **human body louse**. * **Rocky Mountain Spotted Fever (OX19+ and OX2+):** Caused by *R. rickettsii*; transmitted by ticks. * **Limitation:** The Weil-Felix test lacks high sensitivity and specificity; definitive diagnosis is now preferred via Indirect Immunofluorescence Assay (IFA) or PCR.

Question 1286: Bilateral tonsillectomy is performed on an otherwise healthy 11-year-old female with recurrent upper respiratory tract infections. On pathological examination of the tonsils, numerous small, yellow granules are noted. A granule crushed between two slides reveals a dense, gram-positive center and numerous branching filaments at the periphery. The granules are most likely composed of which of the following organisms?

A. Actinomyces israelii (Correct Answer)
B. Aspergillus fumigatus
C. Blastomyces dermatitidis
D. Candida albicans

Explanation: **Explanation:** The clinical presentation and microscopic findings are classic for **Actinomyces israelii**. **1. Why the correct answer is right:** * **Sulfur Granules:** The "small, yellow granules" described are pathognomonic **sulfur granules**. These are actually microcolonies of the bacteria. * **Microscopy:** Crushing these granules reveals a dense core with **Gram-positive, branching, filamentous rods** at the periphery (often described as a "sun-ray" appearance). * **Clinical Context:** *Actinomyces* species are normal commensals of the oral cavity and tonsillar crypts. In patients with recurrent tonsillitis, they can proliferate within the crypts, forming these characteristic granules. While often an incidental finding in tonsillectomy specimens, they can cause cervicofacial actinomycosis (lumpy jaw) following trauma or dental procedures. **2. Why the incorrect options are wrong:** * **Aspergillus fumigatus:** Shows septate hyphae with acute-angle (45°) branching. It is not Gram-positive and does not form yellow sulfur granules. * **Blastomyces dermatitidis:** A dimorphic fungus characterized by large, round yeast cells with **broad-based budding**. It does not form filaments. * **Candida albicans:** Appears as Gram-positive budding yeast and **pseudohyphae**. It does not form organized, macroscopic yellow granules or true branching filaments. **3. High-Yield Clinical Pearls for NEET-PG:** * **Actinomyces** is a **Gram-positive anaerobic bacterium** (not a fungus, despite the name). * **Treatment:** Long-term **Penicillin G** is the drug of choice ("Actinomyces gets Penicillin; Nocardia gets Sulfonamides" – SNAP mnemonic). * **Nocardia vs. Actinomyces:** Both are branching filaments, but *Nocardia* is aerobic and **weakly Acid-Fast (Modified Kinyoun stain)**, whereas *Actinomyces* is anaerobic and not acid-fast. * **IUD association:** *Actinomyces* is also associated with pelvic inflammatory disease in women using intrauterine devices.

Question 1287: How are diphtheria carriers diagnosed?

A. Gram's staining
B. Throat culture (Correct Answer)
C. Albert's staining
D. Schick test

Explanation: **Explanation:** The diagnosis of a **diphtheria carrier** (an individual who harbors *Corynebacterium diphtheriae* without clinical symptoms) relies on the isolation of the organism from the site of colonization, typically the throat or nose. **1. Why Throat Culture is Correct:** Culture is the gold standard for identifying carriers. Swabs are taken from the throat and nose and inoculated onto selective media like **Loeffler’s Serum Slope** (rapid growth) or **Potassium Tellurite Agar** (selective medium where colonies appear black). Since carriers do not have a visible pseudomembrane, culture is the only definitive way to detect the presence of the bacteria. **2. Why Other Options are Incorrect:** * **Gram’s Staining:** This is non-specific. Many commensal "diphtheroids" in the normal throat flora resemble *C. diphtheriae* morphologically, making it impossible to distinguish a carrier from a healthy individual via Gram stain. * **Albert’s Staining:** This is used to demonstrate **metachromatic granules** (Volutin/Babes-Ernst granules). While it helps in presumptive diagnosis of an active case, it cannot definitively confirm a carrier state because some non-pathogenic corynebacteria also possess these granules. * **Schick Test:** This is an obsolete skin test used to determine the **immune status** (susceptibility) of an individual to diphtheria toxin. It does not detect the presence of the bacteria itself. **NEET-PG High-Yield Pearls:** * **Elek’s Gel Precipitation Test:** Used to differentiate toxigenic strains from non-toxigenic strains (essential for confirming a "dangerous" carrier). * **Treatment for Carriers:** Oral **Erythromycin** is the drug of choice to eradicate the carrier state. * **Virulence Factor:** Diphtheria toxin acts by inhibiting **EF-2** (Elongation Factor-2), halting protein synthesis.

Question 1288: Malignant pustules are characteristic of which type of infection?

A. Anthrax infection (Correct Answer)
B. Epithelioma
C. Melanoma
D. Tetanus

Explanation: **Explanation:** **Malignant pustule** is the characteristic clinical lesion of **Cutaneous Anthrax**, caused by *Bacillus anthracis*. Despite the name, it is neither malignant (cancerous) nor a true pustule (as it contains serosanguinous fluid rather than pus). 1. **Why Anthrax is correct:** In cutaneous anthrax, the spores enter through abraded skin. The lesion begins as a painless papule, progresses to a vesicle, and eventually undergoes central necrosis to form a **painless, depressed black eschar** surrounded by significant non-pitting edema. The term "malignant" historically referred to the severity of the untreated infection, which can lead to fatal septicemia. 2. **Why other options are incorrect:** * **Epithelioma & Melanoma:** These are neoplastic (cancerous) conditions. While "malignant" is a term used in oncology, these do not present as "pustules" or acute infectious eschars. Melanoma is a malignancy of melanocytes, and epithelioma refers to tumors of the epithelium. * **Tetanus:** Caused by *Clostridium tetani*, this is a toxemic disease characterized by muscular rigidity and spasms (lockjaw). It does not produce a localized skin lesion like a malignant pustule. **High-Yield Clinical Pearls for NEET-PG:** * **Causative Agent:** *Bacillus anthracis* (Gram-positive, spore-forming, non-motile "Bamboo-stick" appearance). * **Key Feature:** The eschar is characteristically **painless**, which helps differentiate it from pyogenic abscesses or Orf. * **Occupational Hazard:** Also known as **Hide-porter’s disease**, as it commonly affects those handling infected animal products (wool, hides). * **Virulence Factors:** Capsule (poly-D-glutamic acid) and Anthrax toxin (Edema factor, Lethal factor, and Protective antigen). * **McFadyean’s Reaction:** Used for presumptive identification of *B. anthracis* using polychrome methylene blue stain to visualize the capsule.

Question 1289: Stalactite growth in a ghee broth culture is characteristic of which bacterium?

A. Bacillus
B. Legionella
C. Yersinia (Correct Answer)
D. Leptospira

Explanation: **Explanation** The correct answer is **Yersinia pestis**. **1. Why Yersinia is Correct:** *Yersinia pestis*, the causative agent of plague, exhibits a unique growth pattern in liquid media containing a layer of oil or fat. When grown in **Ghee broth** (clarified butter) or broth with oil, the bacteria grow as delicate, hanging filaments or "icicle-like" structures extending downwards from the surface. This is known as **Stalactite growth**. This occurs because the organism is non-motile and prefers the interface between the lipid and the broth for optimal growth. **2. Why Other Options are Incorrect:** * **Bacillus:** *Bacillus anthracis* typically shows a "Medusa head" appearance on agar plates and a "inverted fir tree" appearance in gelatin stabs, but not stalactite growth. * **Legionella:** This is a fastidious organism that requires buffered charcoal yeast extract (BCYE) agar supplemented with L-cysteine and iron. It does not show specific growth patterns in ghee broth. * **Leptospira:** These are spirochetes that require specialized media like EMJH (Ellinghausen-McCullough-Johnson-Harris). They are highly motile and do not form stalactites. **3. High-Yield Clinical Pearls for NEET-PG:** * **Safety Note:** *Y. pestis* is a Tier 1 Select Agent; laboratory diagnosis requires extreme caution (BSL-3). * **Microscopy:** Shows characteristic **"Safety-pin" appearance** (bipolar staining) with Wayson or Giemsa stain. * **Culture:** On blood agar, it forms "fried egg" colonies. * **Vector:** Transmitted by the rat flea (*Xenopsylla cheopis*). * **Virulence Factor:** F1 antigen (capsular) and V/W antigens.

Question 1290: What is the most common sensitive causative organism for Traveller's diarrhoea?

A. E. coli (Correct Answer)
B. Shigella
C. Norwalk virus
D. Rotavirus

Explanation: **Explanation:** **Traveller’s Diarrhoea (TD)** is defined as the passage of three or more unformed stools in a 24-hour period, often accompanied by abdominal cramps, nausea, or bloating, occurring in individuals traveling from developed to developing regions. **Why E. coli is correct:** The most common causative agent worldwide is **Enterotoxigenic *Escherichia coli* (ETEC)**. It accounts for approximately 30–50% of cases. ETEC colonizes the small intestine and produces two types of enterotoxins: **Heat-labile (LT)**, which increases cAMP, and **Heat-stable (ST)**, which increases cGMP. Both lead to the hypersecretion of water and electrolytes, resulting in watery diarrhea. **Analysis of Incorrect Options:** * **Shigella:** While a common cause of bacterial dysentery (bloody diarrhea), it is less frequent than ETEC in travelers. It typically presents with high fever and mucoid stools. * **Norwalk virus (Norovirus):** This is the leading cause of epidemic gastroenteritis and is frequently associated with **cruise ships** and institutional outbreaks, but it ranks behind ETEC for general traveler’s diarrhea. * **Rotavirus:** This is the most common cause of severe diarrhea in **infants and young children** globally, but it is not the primary cause of traveler's diarrhea in adults. **NEET-PG High-Yield Pearls:** * **Most common bacterial cause:** ETEC (Enterotoxigenic *E. coli*). * **Most common parasite:** *Giardia lamblia*. * **Drug of choice (Prophylaxis):** Fluoroquinolones (e.g., Ciprofloxacin) or Rifaximin. * **Mechanism of ETEC LT toxin:** Similar to Cholera toxin (ADP-ribosylation of Gs protein → ↑cAMP). * **Clinical Tip:** If the question specifies "bloody diarrhea in a traveler," consider *Campylobacter* or *Shigella*.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

Practice Questions

Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

Practice Questions

Vibrio, Aeromonas, and Plesiomonas

Practice Questions

Pseudomonas and Related Bacteria

Practice Questions

Haemophilus and HACEK Group

Practice Questions

Bordetella and Brucella

Practice Questions

Mycobacteria

Practice Questions

Spirochetes

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free